There are many different types of flowmeters for measuring velocity of a fluid. The term “fluid” refers gas flow. One category of flowmeter is known as a “thermal” flowmeter. There are two general types of thermal flowmeters. In one type, a flow pipe is employed with a passage for the fluid flow to be measured. One or more electric heaters are located in the flow or the sensor pipe. The heaters apply heat to the fluid as it flows through the sensor pipe. The temperature is measured at two different points in the sensor pipe. The difference in the temperature between the upstream and downstream points can be correlated to velocity.
In the second category of thermal mass flowmeters, a heater/temperature sensor is positioned on a boom and immersed into the flowing fluid stream. A circuit senses the temperature response of the sensor as a function of the mass fluid flow rate.
In each case the rate of heat flow into the fluid from the sensor is directly proportional to the mass flow rate of the fluid. The accuracy of these flowmeters is limited to a relatively narrow range of flow velocities. With low velocities, accuracy is limited by spurious heat losses due to convection and leakage into the environment. For high velocities, accuracy is limited by the finite thermal resistance of the sensor element or elements.
In order to avoid the high velocity range limitation, a common technique is to use a main pipe within which are contained laminar flow elements. These laminar flow elements are arranged to produce a certain pressure drop for the desired range of velocities to be measured. The pressure difference upstream and downstream of the laminar flow elements is proportional to the volume flow rate of the fluid to be measured. A sensor pipe branches off from the main pipe and reenters downstream. This sensor pipe carries heating elements and sensors. A much smaller fluid flow will flow through the sensor pipe than the main pipe. The velocity of the flow in the sensor pipe is measured, it being proportional to the main velocity flowing through the main pipe. In other cases a sensor pipe is not used and instead the sensor elements are positioned within the main duct.
Representative of the art is U.S. Pat. No. 5,419,190 which discloses a method and apparatus for measuring mass flow rates and other characteristics of a fluid in a large conduit uses a sensor circuit having a heater/thermometer, a reference resistor, and a thermometer resistor arranged in four-wire network to enable measurement of all characteristics of the resistor network. That sensor circuit is preferably mounted on an interior wall surface of a support tube. A plurality of such support tubes, each having a plurality of sensors within the support tube, are mounted in an array in the cross section of the duct. A cleaning plate for cleaning the exterior surface of the support tube permits calibration of the network while cleaning. A program for using the circuit to calculate values of interest and to display desirable output data is also disclosed, as is a program for controlling the cleaning/calibration cycle.
What is needed is a gas measurement system for installation in a duct and having a probe comprising a thermal dispersion type sensor and a reference thermistor disposed within a shroud thereon, the shroud having a rectangular entry area for controlling the gas flow past the thermal dispersion type sensor and the reference thermistor. The present invention meets this need.